In a mobile (cellular) communications network, (user) communication devices (also known as user equipment (UE), for example mobile telephones) communicate with remote servers or with other communication devices via base stations. The type of connections that such user equipment may have with the network include: circuit switched (CS) connections, mainly used for voice calls and/or the like; and packet switched (PS) connections, for communicating data packets (e.g. IP packets). Each base station is connected to a core network (such as an IMS or an EPC network), which is in turn connected to other networks for providing end-to-end connectivity for the users.
3GPP specified the IP Multimedia Core Network Subsystem—often referred to as the IP Multimedia Subsystem (IMS)—for enabling IP-based multimedia services for user equipment. A so-called Proxy Call Session Protocol Function (P-CSCE) of an IMS is responsible for exchanging Session Initiation Protocol (SIP) signalling with UEs connected to the IMS. The P-CSCF stores respective UE security and IP session parameters for each connected UE in order to facilitate appropriate encryption of SIP signalling exchanged between a particular UE and the P-CSCF. The P-CSCF is also responsible for authorising the resources necessary for the IP session for each UE and for forwarding messages to and from a next hop CSCF that is appropriate for a particular UE (determined during the registration process performed for that UE).
IMS is specified in 3GPP Technical Specification (TS) 23.228 V13.4.0 and the Session Initiation Protocol (V2.0) is specified in RFC 3261 by the Internet Engineering Task Force (IETF), the contents of which documents are incorporated herein by reference.
IMS Centralized Services (ICS) was specified in 3GPP from Release 8 onwards in several specifications. There are two basic approaches in order to execute supplementary services in IMS for user equipment (UE) that is still using a legacy circuit switched (CS) access towards its Mobile Switching Centre (MSC). One approach is based on enhancements to the MSC, i.e. by making the MSC able to act as a SIP User Agent on behalf of the UE. The other, less preferred approach requires enhancing the operation of the UE in order to make it capable to interact with the Application Server (AS) directly (i.e. without requiring support from the MSC and/or the IMS).
Current solutions include variations of different levels of support of IMS and ICS features in the network (in combination with other networks, if appropriate). One of the more popular options is the so-called MSC-Server (or ‘MSC-S’) that allows Operators to move away from the legacy CS network towards IMS and thus be able to offer IMS services even to legacy UE subscribers. Effectively, the MSC-S is a core network element which controls call switching and mobility management functions for mobile phones roaming in a network of base stations. The MSC-S can also be referred to as an ‘MSC Server’, an ‘MSS’, or an ‘MTS-U’.
When a UE is connected to an (ICS capable) MSC-S using legacy CS protocols, the MSC-S connects to the appropriate IMS entities, e.g. an Application Server (AS) and a CSCF, for handling SIP signalling for the UE. The MSC-S also connects to a Media Gateway for transcoding the CS codecs used by the UE into appropriate IMS codecs. If the UE is a roaming UE, then the MSC-S in the visited network also contacts the home location register (HLR) of the home network in order to download the subscription profile into the visited location register (VLR). Further details on the ICS may be found in: 3GPP TS 23.060 V13.4.0; 3GPP TS 23.228 V13.4.0; 3GPP TS 23.292 V13.2.0; and GSMA IR.64 V12.0, the contents of which documents are incorporated herein by reference.